The present invention relates to circuitry for amplifying signals transmitted through a transmission line and is directed more particularly to circuitry which automatically varies the amplification afforded to signals transmitted through a telephone transmission line in accordance with the a-c losses of that line.
In communication systems wherein a-c signals are transmitted through transmission lines, it is often necessary to provide circuitry which compensates for the attenuation of those signals on transmission through those lines. In some telephone systems, for example, it is necessary to compensate for the attenuation of a-c signals in a loaded transmission line. In these systems, an amplifier circuit, or repeater, provides a relatively flat or fixed gain which when combined with the attenuation characteristic of the transmission line, establishes an overall response of the system including the repeater and the transmission line which decreases the attenuation of signals transmitted therethrough while retaining the desirable frequency response characteristics of the transmission line. One such repeater is shown and described in U.S. Pat. No. 3,706,862 granted in the name of C. W. Chambers, Jr. on Dec. 19, 1972.
Due to the relatively high cost of purchasing and operating a separate repeater for each transmission line, it has been found desirable to operate fixed gain repeaters in a common mode configuration, i.e., switch a relatively small number of repeaters among a relatively large number of occasionally used transmission lines. A problem with switching a fixed gain repeater among several loaded transmission lines is that the attenuation of a loaded transmission line varies in proportion to the length of that line. As a result, the fixed gain provided by that repeater may establish a desirable system response (e.g., 6 db system loss) for a particular length of transmission line, but will be unable to establish that same level of system loss for a different length of transmission line. One attempt to overcome this problem has been the practice to group together loaded transmission lines of similar gauges and lengths in a common mode configuration with a fixed gain repeater which is operable with any member of the group.
Another attempt to overcome the problem associated with switching a repeater among several transmission lines has been the practice of varying the gain of a repeater in accordance with the magnitude of current flow through the transmission line. Circuitry of the above type is shown and described in the U.S. Pat. of Laimons Freimanis, U.S. Pat. No. 3,851,108, entitled "Communication Line Supervisory Circuit." Although the circuitry described in the above-mentioned U.S. Pat. does vary the gain of a repeater in accordance with the line current flowing through the transmission line, that circuitry does not vary the gain of a repeater either in accordance with the a-c losses or the length of that transmission line. This is because the current through the transmission line may be increased due to boost voltages being applied to that transmission line. As a result, the gain established by a repeater of the above type which is connected to a particular length transmission line which does not have boost voltages applied thereto may be less than the gain established by that repeater when connected to the same length of transmission line having boost voltages applied thereto. Since the a-c losses of a transmission line are a function of the length of the transmission line, the circuitry described in the above-mentioned U.S. Pat. is unsatisfactory for use in a common mode configuration wherein transmission lines may have boost voltages applied thereto. Prior to the present invention, loaded transmission lines of differing lengths could not be grouped together and utilized in a common mode configuration.
In telephone systems wherein a repeater is inserted into the transmission line, it is necessary to physically break the transmission line and insert the repeater at this break. Looking into both ends of the transmission line at this break, it may be shown that, ordinarily, the impedance of one section of the transmission line will be different from the impedance of the other section of the transmission line. As a result, the impedance mismatch resulting from the insertion of a repeater into the transmission line will give rise to undesirable echoes of the a-c signal transmitted therethrough.
In order to prevent impedance mismatches from interfering with the transmission of a-c signals through telephone transmission lines, it has been the practice to provide a repeater with passive impedance matching circuits, or line build out networks, which may be adjusted to establish a predetermined impedance (e.g., a 900 ohm resistance) for each section of the transmission line. Circuitry of the above type is shown and described in the U.S. Pat. of R. L. Huxtable, U.S. Pat. No. 2,978,542, entitled "Impedance-Matching Network." When connected to a different length of transmission line, this type of circuitry requires the extensive readjustment of impendances inserted in the line and is, therefore, unsuitable for use with repeaters which are utilized in a common mode configuration.
Another attempt to prevent impedance mismatches from interfering with the transmission of a-c signals through telephone transmission lines has been the utilization of a repeater with active impedance matching circuits, or impedance simulating circuits, which may also be adjusted to establish a predetermined impedance for each section of the transmission line. Circuitry of this type is shown and described in the U.S. patent of Charles W. Chambers, Jr., U.S. Pat. No. 3,828,281, entitled "Impedance Simulating Circuit For Transmission Lines." When connected to a different length of transmission line, this type of circuitry requires readjustment of the magnitudes of the impedance simulating voltages and currents applied to the transmission line and is, therefore, unsuitable for use with repeaters which are utilized in a common mode configuration. Prior to the present invention, neither a line build out network nor an impedance simulating circuit could be utilized with a repeater in a common mode configuration and prevent impedance mismatches from occurring due to the insertion of the repeater into the transmission line.
In accordance with a first embodiment of the present invention, there is provided circuitry which automatically varies the magnitude of amplifying voltages and currents which are applied to a transmission line to establish substantially the same system loss for transmission lines of differing lengths and gauges. In accordance with a second embodiment of the present invention, there is provided impedance matching circuitry which automatically varies the magnitude of impedance simulating voltages and currents which are applied to a transmission line in accordance with the a-c losses of that transmission line to prevent echoes from occurring due to the insertion of a repeater into the transmission line.